The present invention relates to a method for cancelling echoes in a communication system wherein respective signals are sent through a sending path and received through a receiving path, which method comprises the steps of supplying to the receiving path an echo control signal which is derived from the sending path and is representative of an echo to be cancelled in the receiving path.
The present invention also relates to a communication device and to a radio base station of a cordless telephone device.
Such a method and communication device, for example, for a radio base station of a cordless telephone device, is applied in and known from Philips Integrated Circuits Data Sheet, entitled xe2x80x9cPCA 1070, Multistandard Programmable Analog CMOS Transmission ICxe2x80x9d, specifying the PCA 1070 line interface features, and suitable for performing speech and line signal interface functions in electronic telephone sets. In particular the known communication device comprises an analog local-echo-cancelling means, generally also referred to as antisidetone (AST) means, for cancelling local echoes originating from its own local interface, mainly from the so-called hybrid circuit, which performs the separation and combination of respective signals in the sending and receiving paths, respectively, in order to couple a 4-wire telephone interface to the well known 2-wire a/b terminals of a telephone line.
In practise, the aforementioned interface is combined with a digital interface part wherein aside from analog local-echo-cancelling also digital local-echo-cancelling (LEC) is performed for cancelling echoes originating from the nearby interface itself. There is a strong tendency to cancel all kinds of echoes, irrespective of their source, to which the human ear is very sensitive and which may cause irritation and misunderstanding between parties trying to communicate with each other. In particular, echoes are more perceptible and thus more disturbing in present-day high-quality digital European cordless telephone (DECT) systems, due to the roundtrip delay of 20 msec.
It is an object of the present invention to provide an alternative method and a communication device by means of which echoes can be cancelled in an easy-to-implement way and in a way which allows flexible and programmable adjustments to a variety of national authority communication line requirements.
To achieve this, the present invention is characterized in that the echo-control signal is an echo digital control signal which is digital-to-analog converted and then supplied to the receiving path.
Although the proposed invention seems more complicated because of the inclusion of a digital-to-analog convertor (DAC) apart from the generally already present further A/D and D/A convertors in the receiving and sending paths, respectively, it has surprisingly been found that taking an echo representative control signal from the digital sending path, followed by the digital-to-analog conversion and the addition of the converted control-compensation signal to the receiving path, obviates the above disadvantages without adding substantial complexity to the above-mentioned digital software implemented signal processing and programming.
In order to achieve the above objective the echo compensating means comprise an echo digital control signal generating means, and a digital-to-analog converter coupled to the digital control signal generating means and to the receiving path means for compensating echoes.
In addition to the above advantages, the communication device according to the invention gives rise to a possible limitation of the dynamic range of an ADC which is generally also present in the receiving path of the communication device. This dynamic range is a key factor of such a convertor, and an increase thereof would add to the complexity, required chip area and current consumption in an interface IC for communication purposes. The thus achieved limited dynamic range of said ADC provides opportunities for supplying a lower supply voltage to the communication device, which supply voltage is known to limit the dynamic range at its high end in case of a remote PABX, because it is taken from the telephone line itself. At its lower end the dynamic range is limited by the lowest voltage which does not drown in noise. With the limited dynamic range of the ADC, noise requirements are thus advantageously weakened without jeopardising the overall performance of the communication device according to the invention.
Further embodiment of the communication device according to the invention have the advantage that the proposed inclusion or combination of echo digital control signal generating means and/or receiving filter means in the digital signal processing (DSP) means only causes a small and therefore acceptable increase of the DSP software code and execution time, as well as a minor increasexe2x80x94only necessitating a doubled output registerxe2x80x94in DSP hardware.
In the advantageous embodiments of the communication device according to the invention outlined in claims 5 or 6 only one symmetrical digital-to-analog convertor (DAC) is used. This however does not cause a substantial increase in hardware because, for reasons of performance, a symmetrical DAC is preferred anyhow, because of its reduced output noise. Thus, in practise this does not lead to chip area increase or additional current consumption.
The inclusion of a dynamic noise shape (DNS) circuit as proposed in claim 8 only results in a very limited and thus acceptable increase of the chip area in the order of less than 0.05 mm2.